1. Field of the Invention
The present invention relates to an electrophotography type image forming apparatus such as a laser beam printer, a facsimile and a copy machine. More specifically, the present invention relates to an image forming apparatus which charges a surface of a photosensitive drum to a predetermined electric potential by a charger and forms an electrostatic latent image by an exposure, thereby to performing development.
2. Description of the Prior Art
Recently, as an developing method of an electrophotography type image forming apparatus, a so-called impression developing method is broadly used, which utilizes a conductive elastic roller as a developing roller carrying toner and performs development by pressing the developing roller onto a photosensitive drum to perform development.
Specifically, in this developing method, the surface of the photosensitive drum is uniformly electrically charged by a corona electrical charger and then is exposed by an exposing unit to form an electrostatic latent image. Then, the toner is applied to the photosensitive drum surface by the developing roller, and the toner is transferred to the electrostatic latent image side by generating a given developing electric field between the developing roller and the photosensitive drums, thereby carrying out the development.
While this developing method can utilize both positively charged toner and negatively charged toner, the negatively charged toner are primarily used in view of the stability of the electric charge characteristic.
For example, a printer performing the image exposure by a laser beam and the like utilizes a reversal developing method in which the negatively charged toner is used. Additionally, in such a printer, the charging polarity of the photosensitive drum is set to the negative polarity to correspond to the negatively charged toner, and the discharge voltage polarity of the corona electric charging device is also set to the negative polarity. With this arrangement, good print quality is achieved by using the negatively charged toner of stable charge characteristic.
However, there is such a problem that the corona electric charger ionizes oxygen molecules to produce ozone (O.sub.3). There are known that the quantity of ozone produced is substantially proportional to a current flown through the discharge electrodes and that the negative polarity discharge produces more ozone than the positive polarity discharge.
In order to overcome this problem, there is proposed a printer, disclosed in the Japanese Patent Application Laid-Open under No. 5-88587, which uses a distributed organic photosensitive unit of positive charging characteristic and positively charging toner. This printer can use the charger for performing positive polarity discharge as the corona electric charger, and hence this results less ozone production.
However, in the case of using the organic photosensitive unit of positively charging property, the material available for use as the organic photosensitive unit is restricted, compared with the case of using the organic photosensitive unit of negatively charging property, because there are less materials having positively charging property.
Further, in the case of performing reversal development using the positively charging organic photosensitive unit, the positively charging toner should be used. However, the positively charging toner, particularly toner primarily made of polyester, is less reliable in the stability of charging polarity characteristic than negatively charging toner because polyester itself has negatively charging property.
On the other hand, Japanese Patent Application Laid-Open under No. 5-216328 discloses a proposal in which the ozone production is suppressed by reducing the current flown through the discharge electrode of the corona discharging unit using the negatively charging organic photosensitive unit and negatively charging toner.
However, it is known that the negative polarity discharge by the corona electrically charger produces ozone of more than ten times denser than the positive polarity discharge. The electrical charger disclosed in Japanese Patent Application Laid-Open under No. 5-216328 is a Scorotron type charger having grid electrodes. In order to charge the photosensitive unit to a degree necessary to form an electrostatic latent image by the Scorotron type charger, substantial quantity of current, including current flown through the grid electrodes, should be flown through the discharge electrode due to its structure, and hence the charge efficiency becomes low. In this view, there is a limitation in reducing the discharge current in the Scorotron type charger, and the negative polarity discharge of substantial current quantity should be carried out in the end. Therefore, it is still difficult to sufficiently suppress the ozone production.